Paris: Activists from environmental group Greenpeace managed to sneak into a French nuclear power plant on Monday in what they said was a bid to highlight the dangers of atomic energy.

In a statement, Greenpeace said some members had entered the nuclear site at Nogent-sur-Seine, 95 kilometres southeast of Paris, to "spread the message that there is no such thing as safe nuclear power."

Eight activists entered the power station site, according to a French police source, who added that some of the intruders had already been apprehended.

"A group of militants managed to climb on to the dome of one of the reactors, where they unfurled a banner saying 'Safe Nuclear Power Doesn't Exist'," said Greenpeace Spokesman Axel Renaudin.

"The aim is to show the vulnerability of French nuclear installations, and how easy it is to get to the heart of a reactor," said Sophia Majnoni, a Greenpeace nuclear expert.

She denounced a government security audit of French nuclear plants as "a communications exercise which does not take into account risks already identified in the past and does not learn the lessons of Fukushima," the Japanese nuclear plant that was crippled by an earthquake and tsunami.

The Nogent-sur-Seine plant, run by the EDF energy company, was chosen by Greenpeace because it is the nearest plant to Paris, Greenpeace said.

Officials at EDF would not immediately comment on the incident.

French Industry Minister Eric Besson said that if the dawn intrusion was confirmed it would indicate a dysfunction in the plant's security system.

"If an enquiry confirms (the break-in) that would mean that there has been a dysfunction and that measures must be taken to ensure that it doesn't happen again," the minister said in French radio.

"Maybe they succeeded in doing this. That surprises me because our power stations are well guarded," Besson said.

Henri Guaino, an adviser to President Nicolas Sarkozy, said the activists' move was "irresponsible" but raised questions about security at nuclear plants.

"It was irresponsible on their part," he told BFMTV. "But this does make one think about the security of access to nuclear power plants. Conclusions must be drawn from this."

Greenpeace's action came as UN climate talks entered their second week in South Africa.